Vacuum capstans



Oct. 6, 1964 D. T. GWILLIM 7 3,151,795

VACUUM CAPSTANS Filed March 5, 1962 2 Sheets-Sheet 1 D. T. GVVILLIM VACUUM CAPSTANS Oct. 6, 1964 2 Sheets-Shet 2 Filed March 5, 1962 United States Patent 3,151,75 VAQUUM tlAPSTA David Thomas Gwillim, London, England, assignor to Decca Limited, a British company Filed Mar. 5, i962, er. No. 177,477 7 Claims. {61. ZZtS-QS) This invention relates to vacuum capstans having a continuously rotated apertured capstan member within which is a fixed assembly containing a control valve for controlling the application of a vacuum to the apertures in the capstan member. Such vacuum capstans are used, for example, in high speed magnetic tape recording and reproducing apparatus. The capstan member can be continuously driven but will not grip the tape to drive the latter until the vacuum is applied to the apertures and the valve thus provides a means for rapidly starting and stopping the tape without having to start or stop any rotating mechanical member.

According to this invention, in a vacuum capstan having a continuously rotated apertured capstan member within which is a fixed assembly containing a control valve for controlling the application of a vacuum to the apertures in the capstan member, the control valve being an electromagnetically operated change-over valve connecting the apertures to a vacuum chamber or an air chamber within the fixed valve assembly. The fixed assembly may conveniently have a peripheral slot extending in the peripheral direction within the capstan member over the region where vacuum has to be applied to the apertures in the capstan member and said valve member may include an electro-magnetically operated diaphragm movable in the axial direction of the capstan to seal one or other of the longer sides of the slot according to the energisation of one or other of two energising windings and the vacuum chamber and the air chamber within the fixed valve assembly may be arranged, one on each side of the diaphragm, so that one or other of these chambers is opened to said slot according to the position of the diaphragm. it will be seen that by this construction the valve is arranged within the capstan member and the valve seat can be very close to the apertures in the capstan member. Thus the amount of air that has to be drawn out by the vacuum to bring the tape or other driven member into gripping contact with the capstan member or the amount of air that has to be admitted to release the tape can be almost negligible and it is therefore possible to obtain a very high speed of operation.

It will be appreciated that the tape or the like normally lies in contact with the capstan over a limited arcuate extent and the vacuum suction has to be applied only to those apertures in the capstan member which lie within this arcuate extent. The slot in the fixed part of the valve member (or a group of slots if more than one is provided) thus extends only over a limited arcuate region. it is possible therefore to arrange for the diaphragm to tilt about an axis along or tangential to that edge of diaphragm which is opposite to the edge extending into the slot, the diaphragm sealing agains one or other edge of the slot. Most conveniently, however, the diaphragm comprises a plate slidable normal to its plane in the axial direction of the capstan member. The aforementioned air chamber may be connected to a source of pressurised air or may be open to the atmosphere. In some cases the air chamber may be connected to a weak vacuum as the slight vacuum will then serve to keep the tape in close proximity to the capstan when the tape is not being driven.

The aforementioned control valve may comprise a circular diaphragm member of ferro-magnetic material con- 3,l5l,795 Fatented Oct. 6, 1964 stituting said diaphragm, and two energising windings arranged on ferro-magnetic cores, one on each side of the diaphragm, so that the diaphragm is attracted towards one or other of the two sides of the slot according to which of the two windings is energised.

The vacuum chamber and air chamber may be annular chambers around the enerising coils and coaxial with the axis of the valve assembly which conveniently is of cylindrical form. In such a construction each chamber may have concentric inner and outer edges at one end around part of its periphery and the diaphragm may be arranged to seal against both the inner and outer edges of the appropirate chamber when drawn by the magnetic force against the end of that chamber. Preferably an aperture or apertures are provided through the diaphragm member in the region within the inner edges of the chambers for equalising the pressures on the two sides of the diaphragm. Such an aperture or apertures would not alfect the aperation of the device once the diaphragm member has sealed one or other of the two chambers but it will reduce the force required to move the diaphragm and will assist the speedy movement of the diaphragm when changing over in the direction to apply a vacuum to the capstan.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawings in which:

FIGURE 1 is a front elevation partly cut away to show the internal construction of part of a tape drive system employing a vacuum capstan for a magnetic tape reproducing and recording apparatus;

FIGURE 2 is a section along the line 22 of FIG. URE 1 through the capstan assembly employed in the apparatus of FIGURE 1;

FIGURE 3 is a plan view of a valve assembly employed in the capstan assembly of FIGURES l and 2; and

FIGURE 4 is a longitudinal section through the valve assembly of FIGURE 3.

Referring to FIGURES 1 and 2 the tape drive capstan has a continuously rotating capstan member 16 which is of generally cylindrical form. The tape lies over this member within a recess 11 underneath a movable bridge member 12 which is pivotally mounted on a pivot (not shown) so as to be capable of being lifted up away from the capstan member 1t? when the tape is to be threaded through the assembly. The tape in the capstan assembly of FIGURES 1 and 2 extends around the capstan member 15 over an arcuate extent of slightly more than The capstan is driven in the clockwise direction as seen in FIGURE 1 and a stripper 13 is provided to ensure that the tape is guided away from the capstan member at the appropriate point around the periphery thereof. This particular capstan member is arranged for driving the tape in one direction only but the tape may be pulled over this capstan in the reverse direction by another similar capstan turning in the opposite direction.

The rotatable capstan member 10 is mounted on two bearings 14, 15. For convenience of terminology the bearing 14 will be referred to as the rear bearing and the bearing 15, which is nearer the front of the assembly, as the front hearing. The rear bearing 14- supports the capstan member within an outer fixed mounting member 16. The capstan member It extends rearwardly through the bearing 14 and beyond this bearing is secured by a sleeve 17 to a fly wheel 18 and drive system (not shown) for the capstan member 10. The front bearing 15 fits in the open front end of the capstan member 10 and supports the capstan member on a spigot 29 which is a mount for a fixed assembly inside the capstan member and containing a valve assembly 21. The internal construction of the valve assembly 21 will be described in further detail later with reference to FIGURES 3 and 4. For the present it will sutfice to say that the valve assembly 21 has an outer surface of generally cylindrical form. Extending out of the valve assembly are a tube 22 leading to a source of vaccum and a tube 23 enclosing wires 24 for circuits controlling the electro-magnetic operation of the valve. The valve assembly 21 is threaded to receive a bolt 25 which passes through a bore through the spigot20, the head of the bolt bearing against a flange 26 on the spigot 20, which flange overlaps part of a front plate 27 formed integrally with aforementioned fixed member 16 so that the Valve assembly 21 is secured rigidly to the fixed part of the structure of the apparatus. The valve assembly 21 is withdrawable through the back bearing 14 after unscrewing the bolt 25. The aforementioned flange 26 is secured by bolts to the plate 27. Springs 28 passing through apertures in the plate 27 ex-. tend between the inner surface of the flange 26 and a thrust ring 29 to press this thrust ring and hence the front bearing 15 against a shoulder 30 on the inner surface of the capstan member 10. A shoulder 31 on the outer surface of the capstan member applies this axial thrust to a further thrust ring 32 .and thus to the back bearing 14. The springs 28 thus move to take up any axial play in the two bearings 14, 15.

Around the periphery of the capstan membe 10 along the centre line of the base of the groove 11 are a series of apertures 35 (FIGURE 2). These apertures extend radially through the capstan member 10 and extend from the interior of the capstan member into the bases of shallow grooves 36 which extend in the axial direction of the capstan for part of the width of the main groove 11. The tape to be driven sits on the base of the main groove 11 and the shallow grooves 36 serve to distribute the vacuum or air pressure applied through the holes 35 across nearly the whole width of the tape. The grooves 36 and holes 35 are evenly spaced around the whole periphery of the capstan member 10.

The valve assembly is shown in further detail in FIG- URES 3 and 4 and comprises a substantially cylindrical assembly 21 formed by two pot-shaped cores 43, 44 of ferro-magnetic material each having a circular base with an axial central projection 45, the two cores having their bases at opposite ends of the valve assembly. Windings 46, 47 are arranged around the central projections in these two cores leaving annular spaces 48, 49 between the outer faces of the windings 46, 47 and the inner surfaces of annular outer portion 50, 51 of the two cores. One of these annular spaces 49 is connected through a duct 52 to an annular region 53 formed as a groove within an end mounting plate 54 for the valve assembly, this annular groove 53 communicating through a bushing 55 to the aforementioned tube 22 leading to a source of vacuum. The end plate 54 is sealed against the base of the core member 44 by an O-ring seal 56 and a rubber gasket 57. The other annular space 48 is open via holes 58 to the space outside valve assembly and hence to the atmosphere. This annular space 48 could, if desired, be connected to a source of air under pressure if it is required to apply a pressure above atmospheric pressure when the vacuum is removed from the under surface of the tape. In some cases, a weak vacuum might be applied to the annular space 48 so that, when the tape is not being held tightly against the capstan member to be driven thereby, it is held loosely to be kept in close proximity to the capstan member.

A circular diaphragm 60 of ferro-magnetic material is arranged between the two cores 43, 44. This diaphragm 60 is slidable on a sleeve 61 of loaded polytetrafiuoroethylene or similar material within the valve assembly. The diaphragm can seat on the end faces of the outer annular portions 50, 51 of the two cores. In the position shown in FIGURE 4 the diaphragm 60 is seated against the end face of the outer portion 51 of the core 43 and forms an airtight seal around the whole periphery thereof. It is also arranged to seat simultaneously against a seat 62 defining an inner edge to the air chamber 48 and formed of part of a non-magnetic former for the winding 46. Around the greater part of the valve assembly this former extends radially outwardly to meet the outer portion 51 of the core 43. As seen in FIGURE 1 and on the left hand side of FIGURE 4, over an arc of about there is a space between the outer edge of the former and the outer portion 51 of the core 43 and it is this region which is sealed by the diaphragm 69 seating on the outer portion 51 of the core and on the seat 62. Thus the annular air chamber 48 is completely sealed by the diaphragm when in the position shown in FIGURE 4 but the vacuum chamber 49 is open around the end of the outer portion 50 of the core 44. The diaphram 60 would be attracted to and held in this position by energising the winding 46. If the other winding 47 is energised, the-diaphragm 60 would be attracted to seat against the outer end face of the annular portion 50 of the core 44 and also against a seat 63 defining the inner edge of the vacuum chamber 49. The seat 63 is formed as part of a non-magnetic former for the windings 47. The windings 46, 47 are energised selectively by means of the aforementioned leads 24 and it will be noted that the leads to the winding 47 have to be sefled where they pass through the core member 44 as they have to enter into the vacuum chamber 49. The leads to the Winding 46 would also be sealed where they pass through the core member 43 if the chamber 48 is at other than atmospheric pressure. It is desirable to have an air gap in the magnetic circuit and for this reason the inner portions 45 of the two core members 43, 44 do not make contact with the diaphragm member 60 when the latter is attracted to the core.

Apertures 64, typically four apertures dispersed around the axis, are provided in the inner part of the diaphragm member 60 within the seats 62, 63 for equalising the pressures on the two sides of the diaphragm. This not only reduces the force required to move the diaphragm but also enables more rapid movement of the diaphragm to be obtained when the diaphragm is being moved in the direction to apply vacuum to the capstan. This is because, without these apertures, When the diaphragm is being moved from one position to the other, it would have a vacuum across the whole or substantially the whole of one face and atmospheric or possibly higher pressure across the whole or substantially the whole of the other face. This would tend to make the movement of the diaphragm sluggish when changing in the direction to apply a vacuum to the capstan as the pressure on the diaphragm opposes this movement. The apertures 64 equalise the pressures on the two sides of the diaphragm when it is moving from one position to the other. Since these apertures lie within the seats 62, 63, they will not affect the operation of the device once the diaphragm is sealing one or other of the air or vacuum chambers 48, 49.

The two cores 43, 44 are spaced apart, as seen in FIGURE 4, an amount slightly greater than the thickness of the diaphragm member 60, by means of a spacer 72 in which two slots 70, 71 are formed. These slots 70, 71 extend only over the part of the periphery of the assembly over which the tape is to be maintained in contact with the capstan member 10 when the tape is to be driven. It would be possible to have a single slot extending over the whole required arcuate extent of about 90 but, in the construction shown, this arcuate length is divided into two slots to give a mechanically stronger assembly. Around this part of the valve assembly is a carbon member 73 the surface of which is slightly proud of the general cylindrical surface of the assembly. The slots 70, 71 are aligned with a slot through the member 73 extending around the whole of the required are as seen in FIGURE 3. The member 73 bears against the inner surface of the rotatable capstan member10. The slots 70, 71 lie in the same radial plane as'the aforementioned holes 35 (FIGURE 2) so that, as these holes pass over the slots'in the valve assembly, either vacuum or air pressure is applied through the holes 35 to the underside of the tape according to the position of the diaphragm member 60. The capstan member 10 can rub against the carbon member 73 to provide a substantially airtight seal for the connection between the slots in the valve assembly and the aforementioned holes 35. Provision is made for effecting a fine adjustment of the radial position of the member 73 with respect to the capstan member 10, an eccentric head 75 (FIGURE 2) on the end of a rotatable shaft 76 engaging a recess 77 in the valve assembly 21 so that, after sIackening the bolt 25, by turning the shaft 76, for example by means of a screwdriver engaging a slot in the outer end of the shaft, the whole valve assembly may be moved slightly towards or away from the inner surface of the capstan member. After such adjustment, the bolt 25 is tightened to hold the valve member in position.

It will be seen that by the application of current to one or other of the two windings 46, 47 the diaphragm 64 can be moved to connect either the source of vacuum or atmospheric pressure to the slots 70, 71 and hence to the underside of the tape. The capstan member is continuously driven but only grips the tape when the vacuum is applied and the valve thus provides means for rapidly starting and stopping the tape under electrical control without having to start or stop any rotating mechanical members. The only mechanical movement required is the very small movement of the diaphragm member 69 and, because of the very short distances involved a negligible quantity of air needs to be extracted when the vacuum is applied or to be admitted when the vacuum is released so enabling a very high speed of operation to be obtained.

I claim:

1. A vacuum capstan comprising a rotatable apertured capstan member and a fixed assembly Within the capstan member containing a control valve, said fixed assembly having a slot extending in the circumferential direction within the capstan member with a sealing element of softer material than the capstan member around the periphery of the slot bearing against the inner surface of said capstan member, and said valve including a diaphragm of ferro-magnetic material extending into said slot and movable in the axial direction of the capstan to seal against one or other of the longer sides of the slot according to its axial position, a vacuum reservoir on one side of said diaphragm, an air chamber on the other side of said diaphragm, two solenoid windings arranged one on each side of said diaphragm, and means for selectively energizing either one of the windings.

2. In a tape drive system, a vacuum capstan for selectively engaging the portion of tape extending over an arcuate region of the capstan comprising a rotatable continuously driven capstan member having a series of apertures around the periphery of the capstan member to lie under the center line of the tape, transverse grooves extending parallel to the axis of the capstan member on the outer surface thereof, which grooves each open into one of said apertures and extend symmetrically therefrom with a total length less than the width of the tape, a stator assembly in said capstan member, which stator assembly comprises a cylindrical housing with its axis parallel to the axis of the capstan member and having an arcuate slot in said housing extending in the circumferential direction within said capstan member, a sealing element on said housing around said slot, said stator being positioned with said sealing element contacting the inner surface of the capstan member, a diaphragm extending into said slot and movable in the axial direction of said capstan to seal against either one or the other of the longer edges of the slot according to the axial position of the diaphragm, two solenoids one on each side of the diaphragm and each arranged when energized to attract the diaphragm, means for selectively energizing either one of the solenoids, and a vacuum reservoir on one side of the diaphragm and an air chamber on the other side of the diaphragm, said reservoir and chamber extending around at least the peripheral portions of the diaphragm adjacent said slot.

3. In a tape drive system, a vacuum capstan for selectively engaging the position of tape extending over an arcuate region of the capstan comprising a rotatable continuously driven capstan member having a series of apertures around the periphery of the capstan member to lie under the center line of the tape, transverse grooves extending parallel to the axis of the capstan member on the outer surface thereof, which grooves each open into one of said apertures and extend symmetrically therefrom with a total length less than the width of the tape, a stator assembly in said capstan member, which stator assembly comprises a cylindrical housing with its axis parallel to the axis of the capstan member and having an arcuate slot in said housing extending in the circumferential direction within said capstan member, a sealing element on said housing around said slot, adjusting means for adjusting the position of said stator assembly in said capstan member to bring said sealing element into contact with the inner surface of the capstan member, a diaphragm extending into said slot and movable in the axial direction of said capstan to seal against either one or the other of the longer edges of the slot according to the axial position of the diaphragm, two solenoids one on each side of the diaphragm and each arranged when energized to attract the diaphragm, means for selectively energizing either one of the solenoids, and a vacuum reservoir on one side of the diaphragm and an air chamber on the other side of the diaphragm, said reservoir and chamber extending around at least the peripheral portions of the diaphragm adjacent said slot.

4. A vacuum capstan as claimed in claim 3 wherein said sealing element is formed of carbon.

5. A vacuum capstan comprising a rotatable apertured capstan member and a fixed assembly Within the capstan member containing a control valve, said fixed assembly having a slot extending in the circumferential direction within the capstan member with a sealing element of softer material than the capstan member around the periphery of the slot bearing against the inner surface of said capstan member, and said valve including a diaphragm of ferro-magnetic material extending into said slot and movable in the axial direction of the capstan to seal against one or other of the longer sides of the slot according to its axial position, two solenoids arranged one on each side of said diaphragm, means for selectively energizing either one of the solenoids for effecting axial dispacement of said diaphragm, a vacuum chamber on one side of the diaphragm and an air chamber on the other side of said diaphragm, said chambers being of annular form surrounding said solenoids and each chamber having concentric inner and outer edges at the end nearer the diaphragm around part of its periphery, the diaphragm being arranged to seal against both the inner and outer edges of the appropriate chamber when drawn by the magnetic force of a solenoid against the end of that chamber.

6. A vacuum capstan as claimed in claim 5 wherein at least one aperture is provided through the diaphragm in the region within the inner edges of the annular chambers for equalizing the pressures on the. two sides of the diaphragm while it is moving.

7. In a tape drive system, a vacuum capstan comprising a rotatable apertured capstan member containing a control valve, said fixed assembly having a slot extending in the circumferential direction within the capstan member with a sealing element of softer material than the capstan member around the periphery of the slot hearing against the inner surface of said capstan member, and said valve including a diaphragm extending into said 7 a a a slot and movable in the axial direction of the capstan to seal against one or other of the longer sides of the slot according to its axial position, solenoid means selectively energizable to eflect movement of the diaphragm in either direction, a vacuum chamber on one side of the diaphragm and an air chamber on the other side of the diaphragm, said chambers having open ends facing said diaphragm and the diaphragm being arranged to seal the open end.

of the appropriate chamber when moved towards that chamber, and said diaphragm further having at least one aperture through the diaphragm in a part of the diaphragm remote from the open ends of said chambers for equalizing the pressures on the two sides of the diaphragm while it is moving.

References Cited in the file of this patent UNITED STATES PATENTS Baumeister et al Oct. 4, 1960 Faeber Dec. 19, 1961 Wardell Nov. 27, 1962 FOREIGN PATENTS Great Britain Feb. 17, 1960 

1. A VACUUM CAPSTAN COMPRISING A ROTATABLE APERTURED CAPSTAN MEMBER AND A FIXED ASSEMBLY WITHIN THE CAPSTAN MEMBER CONTAINING A CONTROL VALVE, SAID FIXED ASSEMBLY HAVING A SLOT EXTENDING IN THE CIRCUMFERENTIAL DIRECTION WITHIN THE CAPSTAN MEMBER WITH A SEALING ELEMENT OF SOFTER MATERIAL THAN THE CAPSTAN MEMBER AROUND THE PERIPHERY OF THE SLOT BEARING AGAINST THE INNER SURFACE OF SAID CAPSTAN MEMBER, AND SAID VALVE INCLUDING A DIAPHRAGM OF FERRO-MAGNETIC MATERIAL EXTENDING INTO SAID SLOT AND MOVABLE IN THE AXIAL DIRECTION OF THE CAPSTAN TO SEAL AGAINST ONE OR OTHER OF THE LONGER SIDES OF THE SLOT ACCORDING TO ITS AXIAL POSITION, A VACUUM RESERVOIR ON ONE SIDE OF SAID DIAPHRAGM, AN AIR CHAMBER ON THE OTHER SIDE OF SAID DIAPHRAGM, TWO SOLENOID WINDINGS ARRANGED ONE ON EACH SIDE OF SAID DIAPHRAGM, AND MEANS FOR SELECTIVELY ENERGIZING EITHER ONE OF THE WINDINGS. 